The projection television set has a projection lens system in front of each of blue, green and red cathode ray tubes to project the picture images from the respective cathode ray tubes on a screen which is located forward of the projection lens system, thereby forming color pictures on the screen.
The projection lens systems to be used in such projection television sets employ glass or plastic lens elements. In the projection lens system of this sort, it has been generally considered that positive correction of chromatic aberration is unnecessary in view of the narrow width of emission spectrum of the fluorescent material. However, recently there is a strong demand for projection lenses capable of correcting chromatic aberration to a sufficient degree to cope with the trend toward high quality pictures, particularly for high resolution projection lenses for use with a large screen or a high definition display.
In this connection, in a case where the lens components of a projection lens system are all made of plastic, the correction of monochromatic aberration is possible by using aspheric lenses which can be easily formed by injection molding. However, in the current state of the art which permits only a narrow freedom in selecting the plastic lens material, there are still problems such as insufficient correction of chromatic aberration and shifts of imaging point which occur to an objectionable degree under varying temperature condition as a result of changes in the refractive index of the plastic which has large thermal coefficient and linear expansion coefficient. Conversely, the correction of chromatic aberration as well as the correction for the shift of imaging point with temperature is easy in a case where the lens components are all made of glass lenses, but it is necessary to employ six or seven lens elements in order to correct the various aberrations while ensuring high quality picture. This is, however, extremely disadvantageous in terms of cost and weight.
In this connection, there has been proposed the so-called hybrid type projection lens using a combination of plastic and glass lenses, for example, as described in U.S. Pat. No. 4,761,063. This prior art projection lens system is composed of five lens units each consisting of a single lens element, consisting of, from the screen side, a positive first lens, a negative second lens, a positive third lens, a positive fourth lens, and a negative fifth lens, of which the third lens is of glass, the first, fourth and fifth lenses are of acrylic, and the second lens is of polystyrene. This arrangement employs a positive acrylic lens for the first lens and a negative polystyrene lens for the second lens in an attempt to correct the chromatic aberration as well as the shift of imaging point with temperature, utilizing the differences in temperature dependent variations of Abbe number and refractive index between the two kinds of plastic lenses.
A lens system of such a construction, in addition to the reduction of the number of glass lens which is disadvantageous weight- and cost-wise, can contribute to some extent to the correction of chromatic aberration and improvement of the shift of imaging point with temperature. However, simply using the first and second lenses of different materials has a limit in correcting the chromatic aberration and the shift of imaging point with temperature, and cannot correct the shift of imaging point caused by components other than lenss, for example, by thermal expansion of the lens barrel or the like, coupled with insufficiency of corrections for errors other than the chromatic aberration. Therefore, it is still far from replying to the demands for high quality pictures as a wide screen projection lens or a high resolution projection lens for high definition television. Further, of the plastic lenses except the fifth lens which is located most closely to CRT, the first, second and fourth lenses which are allotted with a certain degree of lens power have a large difference in thickness between center and peripheral lens portions. These plastic lenses of this sort are normally manufactured by injection molding, but such a large difference in thickness hinders improvements in lens accuracy. Namely, due to the large difference in thickness, non-uniform contraction is like to occur in the cooling stage subsequent to the injection molding, resulting in distortion of the lens structure.